ABSTRACT: Aquatic sediments host much of the bacterial biomass and biodiversity and play a key role in biogeochemical cycles. However, the potential effect of viral infection remains unknown. We present estimates of virus production in a variety of benthic habitats of the Mediterranean Sea, characterized by different contamination levels and trophic states. Viriobenthos abundance in aquatic sediments ranged 108-109 ml-1 of sediment and was ~20 times higher than virioplankton abundance. Vertical profiles in sediment cores revealed large virus numbers at 1-m depth below the sediment surface. Virus production in both marine and freshwater sediments was 1-2 orders of magnitude higher than typical values reported for virioplankton (0.13-1.60 x 108 virus-like particles ml-1 sediment h-1), which indicates that virus turnover and infection rates can be higher in aquatic sediments than in the water column. Eutrophic and contaminated sediments displayed the highest virus production rates. Virus production was significantly correlated with the abundance of active bacteria and with bacterial cell production. We estimated an average burst size (BS) of 15-18 in marine and freshwater sediments. Viru-mediated bacterial mortality was high (on average, >40% of bacterial production) and increased from surface to deeper sediment levels, down to 100-cm depth. The fraction of bacteria with lysogenic infection (range, 0.0-1.8%) increased in deeper sediment layers (3.3% at 100-cm depth). Our results suggest that high benthic virus production rates can have a significant effect on benthic bacterial dynamics and indicate that virus production should be included in biogeochemical models of aquatic sediments.

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